Wireless power transmission technologies include a method of using a radio frequency (RF) and a method of using a magnetic field. Examples of the method of using a magnetic field may include a magnetic field induction-based non-contact power transmission method, a magnetic field beam shaping-based short-distance power transmission method, and a magnetic field resonance-based short-distance power transmission method.
Non-contact methods have been increasingly used in the fields of portable electronic devices, portable terminals, charging pads, etc. Such methods are expected to be applied to more portable devices, but are disadvantageous in that a power transmission distance is very short.
In a magnetic field resonance-based wireless power transmission technique, a transmitter and a receiver are designed to resonate with each other, thereby greatly increasing transmission efficiency, but transmission efficiency decreases as the distance between the transmitter and the receiver increases.
The method of using an RF has been used very frequently, for example, in the form of radio frequency identification (RFID). However, a maximum power transmission rate is very low and whether results of conducting a clinical test and a verification test are harmless is not confirmed. It therefore demands a physiological approach.
The magnetic field beam shaping-based short-distance power transmission method is available for electric cars and electric rail cars, but it is not easily applicable to small-sized portable electronic devices since ferrite which is large in size and heavy in weight is used.
The above methods are techniques using an electric field, electromagnetic waves, and a magnetic field, based on Faraday's law, but need to be improved in terms of efficiency and directivity.
Furthermore, a power transmission method using a general ultrasonic transmission device or an array of ultrasonic transmission devices is disclosed in U.S. Pat. No. 6,798,716 B1, in which an array of ultrasonic transmission devices are provided, a beam is focused on a central point by controlling the phases of the ultrasonic transmission devices, and the direction of the beam is adjusted through phase control. Korean Laid-Open Patent Application No. 10-2012-0073973 discloses an ultrasonic transmission device in which a space is formed between an upper plate and a lower part of a radiation plate to increase a natural frequency so that ultrasonic waves may be emitted in a higher order mode, thereby decreasing the thickness of the radiation plate and increasing the hardness thereof. Korean Laid-Open Patent Application No. 10-2013-0119837 discloses an ultrasonic charging power supply module and a polyhedral ultrasonic charging power supply device having the same. Here, a circuit board which converts electric energy obtained through conversion by an ultrasonic reception device into predetermined intensity electric energy; and a secondary battery formed in a package and configured to store the electric energy obtained by the circuit board are provided. The package, a flexible hinge, and a reception vibration plate are formed of a titanium alloy. However, more research is needed to improve this technique so that it may be applicable to portable devices which are required to be embodied in many different forms and manufactured in small sizes.
Furthermore, this technique is limited to a technique of wirelessly transmitting electric energy into a human body from the outside using ultrasonic waves which easily penetrate human tissue such as skin, a muscular layer, or a layer of fat. Thus, this technique is not easily applicable to short-distance wireless power transmission and is expected to have difficulties.
Ultrasonic waves on which ultrasonic power transmission technology will be based hereinafter have a feature that refraction occurs between media due to the difference between propagation velocities of the media. Table 1 of FIG. 6 shows degrees of refraction and reflection in air and water.
The feature of the ultrasonic waves is based on Huygen's principle (sin θ1/sin θ2=V1/V2=λ1/λ2=n). Here, since V2 is higher than V1, θ2 is greater than θ1. In this case, when an angle of incidence θ1 is decreased toward water, incident waves may not be capable of entering the water (θ2=90).
As shown in Table 1 of FIG. 6, ultrasonic waves are waves which can be easily reflected. A distance and a thickness may be measured using an ultrasound sensor and ultrasonic waves, based on this feature of ultrasonic waves.
A material of a piezoelectric element will be additionally described with reference to Table 2 of FIG. 6 below. PMN-PT is a solid-state single crystal of lead magnesium niobate (PMN) which is a relaxor and lead titanate (PT) which is a piezoelectric material. PMN-PT is a high-performance relaxor-based piezoelectric single crystal material which has three or more times the piezoelectric distortion of lead zirconium titanate (PZT) ceramics generally used as a piezoelectric material. When voltage is applied to a PZT material in a short pulse form, the PZT material vibrates and generates ultrasonic waves.
As shown in a table of FIG. 7, values d33 and d31 (piezoelectric constants) and a value K33 (electro-mechanical coupling factor) which determine features of a piezoelectric material of PMN-PT single crystal are far greater than those of an existing PZT material. Thus, the physical properties of the PMN-PT single crystal exhibit a higher effect when it is applied to a device using the PMN-PT single crystal.
(1) Electro-Mechanical Coupling Factor K33
The electro-mechanical coupling factor K33 is a coefficient representing conversion efficiency between electric energy and mechanical energy.
As the electro-mechanical coupling factor K33 increases, a device may be manufactured to have a smaller size and consume lower power.
(2) Piezoelectric Constants d33 and d31
The piezoelectric constants d33 and d31 represent a degree of displacement when an electric field (V/m) is applied or a degree of non-displacement in this case. As the piezoelectric constants d33 and d31 increase, finer displacement control may be performed.
Furthermore, the PMN-PT has a piezoelectric constant d33 which is three or more times that of lead zirconium titanate (PZT) ceramics generally used as a piezoelectric material, and has a higher electro-mechanical coupling factor K33 and exhibits higher piezoelectric characteristics than lead zirconium titanate (PZT) ceramics.
With use of a piezoelectric material, electric energy may be converted into mechanical energy and vice versa.
A general micro-electromechanical system (MEMS) is formed of film layers of silicon and lead zirconium titanate (PZT) which are standard materials of a semiconductor electronic device. Using recent technology employing a lead magnesium niobate-lead titanate (PMN-PT) thin-film combined with a silicon material, power may be locally supplied to a node of a wireless sensor of an energy harvesting device or a sensor applicable to a human body.
In addition, basically, ultrasonic waves use a different frequency band according to the type of medium which the ultrasonic waves penetrate. In general, a frequency band of ultrasonic waves used in a medical instrument ranges from 1 MHz to 20 MHz. As a frequency of the ultrasonic waves becomes higher, the resolution of an image is higher but the ultrasonic waves attenuate faster. Thus, the ultrasonic waves cannot deeply penetrate a human body. Thus, high-frequency ultrasonic waves are mainly used to monitor blood vessels or the like. When low-frequency ultrasonic waves are used, the resolution of an image is low but the ultrasonic waves can deeply penetrate a human body. Thus, the low-frequency ultrasonic waves are used for imaging equipment in the fields of obstetrics and gynecology. A 40 kHz ultrasonic transducer generally used as a distance measurement device operates at a skywave band of very low-frequency ultrasonic waves penetrating even air among ultrasonic waves. Actually, ultrasonic waves output from a medical ultrasonic modulator (transducer) of several MHz cannot penetrate air. Thus, when an image is captured, an air layer is prevented from being generated by using gel. Furthermore, when a frequency band becomes higher, impedance of a piezoelectric element also increases and thus a voltage to be applied should be high.